128 



CELL HEREDITY 



Parental 

 DNA 



Replicating 

 DNA 



Transduced 

 DNA fragment 



FIGURE 5.9. A possible copy-choice mechanism for the replacement of recipient 

 by donor genes after transduction. 



another type. Thus the coat of a transducing phage may enclose several 

 kinds of DNA and, in fact, transducing phages can be separated from 

 others by centrifugation on account of their different density. But the 

 nonphage DNA never includes as much as the whole bacterial geno- 

 type, only fragments of it. We might have anticipated this because, 

 during phage development, the host DNA degrades, some of it even to the 

 level of purine and pyrimidine bases. We have further assurance from 

 the pattern of transduction itself. 



When temperate phages are grown on a strain of the bacterium. 

 Salmonella typhimurium, that is able to synthesize methionine, utilize 

 the sugars galactose and xylose, and resist streptomycin {met'^ gal'*' xyl^ 

 str-r), and are then allowed to infect a strain which has the opposite 

 characters {met~ gal~ xyV str-s), some of the latter survive and are 

 transduced. But such cells are transduced for only one character (Figure 

 5.8). When, for example, selection is made on agar with streptomycin, 

 str-r transductants are found, but they are still met~ gal~ xyl~, and so 

 on. It was the same with transformation in pneumococcus; penicillin- 

 resistance and streptomycin-resistance were conferred independently upon 

 the transformed cells. Both in transformation and in transduction most 

 genes are transferred independently; the pieces transferred are parts, and 

 not the whole, of the genotype. But if two genes are closely linked, they 

 may be transduced together with a frequency that is a function of the 

 closeness of their linkage. On this principle bacterial genes can be 

 mapped by transduction, and their order is found to be that observed 

 by usual crossing techniques. Another method of mapping utilizes only 

 the relative frequencies with which various mutants will transduce one 

 another (Chapter 6). 



It may be supposed that the transduced DNA pairs with the host DNA 



